Apparatus for controlling data transfer operations between a memory and devices having respective latencies

ABSTRACT

An apparatus is described for controlling data transfer operations between a main memory and other devices in a computer system. A memory controller receives data transfer request signals and associated latency identification values, each corresponding with a maximum time interval in which to service the respective data transfer requests. The latency identification values are periodically modified and compared to indicate the current highest priority request. In the event that service of a particular requested data transfer operation must be provided imminently, priority override circuitry is provided. In this way, those devices having particular latency requirements can be provided with timely access to the main memory, and need not have separately dedicated memory or buffers.

TECHNICAL FIELD

This invention relates generally to controlling data transfer between a memory and a device in a computer system, and more particularly, to prioritizing service of multiple data transfer requests.

BACKGROUND OF THE INVENTION

In a typical computer system, multiple devices are able to write data to and read data from a main memory. A memory controller couples these devices with the main memory and controls the timing and sequence of such data transfer operations. Referring to FIG. 1, a memory controller 10 includes arbiter circuitry 12 that receives a plurality of signals indicative of requests for data transfer operations-commonly known as priority requests PREQ -PREQ M. Each of these priority request signals corresponds to a device requesting to write data to or read data from a main memory 14. Example devices include a microprocessor and input/output (I/O) devices, such as peripheral component interconnect (PCI) bus devices, industry standard architecture (ISA) bus devices, integrated drive electronics (IDE) devices, accelerated graphics port (AGP) devices, small computer system interface (SCSI) devices, and universal serial bus (USB) devices, to name just a few examples. The arbiter 12 prioritizes the various priority requests, and the memory controller 10 correspondingly initiates and controls the data transfer operations.

Typically, the priority requests are simply queued by the arbiter 12, with the requested data transfer operations occurring on a first-come first-served basis. Certain of today's computer systems can reorder some data transfer operations, such as by providing “read around write” capability. Also, certain computer systems allow various write operations to be reordered or combined to minimize the frequency of write operations to the memory subsystem. Some computer systems provide fixed service priorities, in which priority requests from some devices (e.g., the CPU) always receive service prior to priority requests from other devices.

Many devices included in today's computer systems have maximum latency requirements—i.e., a maximum time interval following the requested data transfer operation by which such operation must be effected. For example, a video display device typically requires regular refresh of the display. The display refresh must occur regularly and timely to avoid undesirable artifacts showing on the display. As another example, a software-based modem requires timely transfer of coder/decoder (CODEC) data in order to avoid corruption of the modem data. To satisfy such latency requirements, today's computer systems include separately dedicated memories or device buffers. For example, the video display in today's computer systems commonly has a separately dedicated video memory and associated video memory controller. As another example, software-based modems have a large CODEC data buffers for storing received data and transmit data.

Given the improved production economies and performance improvements offered by today's increasingly integrated computer systems, it is highly desirable to minimize the number of separately dedicated memory circuits and associated controller circuits. However, today's main memory access arbitration schemes cannot satisfy the latency requirements of certain devices commonly included in a computer system.

SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus is provided for controlling data transfer operations between a memory and a device in a computer system. The memory stores data, and the device is operable to assert a data transfer request signal. A memory controller couples the memory with a device and controls data transfer operations therebetween. The memory controller receives the data transfer request signal and also receives a latency identifier value corresponding with a maximum time interval for servicing the data transfer request. The memory controller then initiates data transfer operations prior to elapse of the maximum time interval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram depicting a memory and a memory controller in accordance with the prior art.

FIG. 2 is a functional block diagram depicting a computer system in accordance with an embodiment of the present invention.

FIG. 3 is a functional block diagram depicting certain details of a system controller included in the computer system shown in FIG. 2.

FIG. 4 is a functional block diagram depicting certain details of a memory controller included within the system controller shown in FIG. 3.

FIG. 5 is a signal timing diagram depicting the sequence and timing of certain signals applied to and produced by the memory controller shown in FIG. 4.

FIG. 6 is a part functional block, part schematic diagram depicting a portion of an arbiter circuit included within the memory controller shown in FIG. 4.

FIG. 7 is a functional block diagram depicting a memory control state machine included within the memory controller shown in FIG. 4.

FIG. 8 is a state diagram depicting the operations of the memory control state machine shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a novel apparatus is described for controlling data transfer operations between a memory and a device in a computer system. Certain specific details are set forth to provide a sufficient understanding of the present invention. It will be clear, however, to one skilled in the art, that the present invention may be practiced without these details. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention.

FIG. 2 shows a computer system 20 in accordance with an embodiment of the present invention. A microprocessor 22 is coupled with a system controller 26 by a processor bus 24 that carries address, data, and control signals therebetween. The system controller 26 includes a memory controller 28 for accessing a main memory 30 via a memory address/control bus 32 and a memory data bus 34. As understood by those skilled in the art, the address/control bus 32 may itself be separate, parallel address and control signal paths, or the address and control signals may be provided serially, or in some other suitable combination. The memory 30 may include any of a wide variety of suitable memory devices. Example memory devices include dynamic random access memory (DRAM) devices such as synchronous DRAMs, SyncLink DRAMs, or Direct RAMBUS DRAMs.

The system controller 26 also functions as a bridge circuit (sometimes called a North bridge) between the processor bus 24 and a system bus, such as I/O bus 36. The I/O bus 36 may itself be a combination of one or more bus systems with associated interface circuitry (e.g., AGP bus and PCI bus with connected SCSI and ISA bus systems). Multiple I/O devices 38-46 are coupled with the I/O bus 36. A data input device 38, such as a keyboard, a mouse, etc., is coupled with the I/O bus 36. A data output device 40, such as a printer, is coupled with the I/O bus 36. A visual display device 42 is another data output device that is coupled with the I/O bus 36. A data storage device 42, such as disk drive, tape drive, CD-ROM drive, etc., is coupled with the I/O bus 36. A communications device 46, such as a modem, local area network (LAN) interface, etc., is coupled with the I/O bus 36. Additionally, expansion slots 48 are provided for future accommodation of other I/O devices not selected during the original design of the computer system 20.

Although FIG. 2 depicts the various I/O devices 38-46 as being coupled with the system controller 26 via a single shared I/O bus 36, one or more of the I/O devices may have a separately dedicated interface connection to the system controller 26. Alternatively, one or more of the I/O devices 38-46 may be coupled with the system controller 26 via a multiple bus and bridge network. As a further alternative, one or more of the I/O devices 38-46 may be coupled with the system controller 26 partly through a shared bus system and partly through separately dedicated signal line connections. Indeed, those skilled in the art will understand the depiction of FIG. 2 to encompass any of a wide variety of suitable interconnection structures between the memory 30, the memory controller 28, and the I/O devices 38-46.

FIG. 3 shows the connection between the memory controller 28 and a representative I/O device 50 at a device interface 52 integrated within the system controller 26. As mentioned above, the device interface 52 may be a separately dedicated connection for the particular I/O device 50, a shared bus interface, or other suitable signal interface circuitry. When the I/O device 50 desires access to the memory 30, the device interface 52 applies a data transfer request signal REQ to the memory controller 28, as is done in conventional computer systems. The memory controller 28 includes arbiter circuitry 54 that receives the REQ signal. In contrast with conventional computer systems, the arbiter 54 also receives a latency identification value corresponding to the particular I/O device 50. The latency identification value may itself be provided by the I/O device 50, may be hard-coded into the device interface 52, or may be provided as a software ID in system configuration space, as will be understood by those skilled in the art. The memory controller 28 also receives an address corresponding to the location in the memory 30 that the I/O device 50 wishes to access.

The latency identification value provides the arbiter 54 with information concerning the maximum time interval during which the requested data transfer operation must be performed. As will be described in detail below, the memory controller 28 and arbiter 54 ensure that the requested data transfer operations occur within the maximum allowable latency period. Thus, in accordance with embodiments of the present invention, I/O devices with particular latency requirements need not have separately dedicated memory subsystems or buffers to ensure timely satisfaction of their data flow rate requirements.

FIG. 3 depicts only the single representative interface 52 between the memory controller 28 and the single representative I/O device 50. Those skilled in the art will understand that a plurality of such interfaces may be provided to handle the wide variety of data transfer operations between the main memory 30 and a plurality of other devices included within the computer system. Alternatively, a single interface may provide much of the signal routing for multiple devices, but with separate latency identification values for each of the devices using the interface.

FIG. 4 shows the memory controller 28 and arbiter 54 receiving a plurality of data transfer request signals REQ -REQ N, together with associated latency identification values Latency -Latency N and memory address locations Address -Address N. The particular embodiment depicted in FIG. 4 also shows the memory controller 28 receiving a priority request signal PREQ and associated Priority Address. Thus, the memory controller 28 can include conventional priority arbitration as well as the novel latency-tagged arbitration. In the particular embodiment depicted in FIG. 4, the logic circuitry associated with conventional priority request arbitration schemes is located “upstream” from the memory controller 28. Thus, the priority request signal PREQ and associated Priority Address shown in FIG. 4 represent a single priority request passed from another arbitration circuit (not shown) ordering a plurality of priority requests with conventional circuits and methods. Of course, conventional arbitration circuitry could itself be integrated within the arbiter 54, in which case the arbiter would receive both conventional priority requests PREQ -PREQ M and the latency-tagged requests REQ -REQ N with associated latency values Latency -Latency N.

The memory controller 28 asserts respective acknowledge signals ACK[0:N] to the device interfaces 52, and a priority acknowledge signal PACK to the conventional priority arbitration circuitry or device interface, to acknowledge the particular data transfer request being serviced and to initiate associated data transfer operations. Referring to FIG. 5, a timing diagram depicts the corresponding signal sequence and timing. A data transfer request signal is depicted as REQ x, which may be any of the above-described latency-tagged requests or conventional priority request (PREQ). The request signal REQ x will remain asserted until service of the data transfer request has begun, which the memory controller 28 indicates with an asserted acknowledge pulse ACK x (or PACK). The memory controller 28 also provides the various wellknown memory control signals necessary to accomplish the requested data transfer operation, which are depicted in the timing diagram of FIG. 5 as MEMCYCLE x.

As described above, a memory controller 28 in accordance with the present invention receives a data transfer request signal and an associated latency identification value. The memory controller 28 is free to service other requests for data transfer operations subject to the requirement that the latency-tagged request is serviced within a maximum time interval corresponding to the latency identification value. Those skilled in the art will be able to implement such a memory controller and associated arbitration scheme in any of a number of suitable ways. One such implementation is described in connection with FIGS. 6-8. For purposes of simplicity, FIGS. 6-8 show arbitration between two latency-tagged requests REQ  and REQ I and a single conventional priority request PREQ. Those skilled in the art will understand how to extend the implementation to the general case of N latency-tagged requests and M priority requests.

Referring to FIG. 6, a synchronous down counter 60 loads the Latency  value at a counter input IN and produces a decremented latency value at a counter output OUT. Signals derived from the request signal REQ  are applied to Preset and Load inputs of the counter 60. When REQ  is deasserted, an inverter 62 then applies a corresponding asserted Preset signal, thereby maintaining the output produced by the counter 60 at a maximum, nondecrementing level. When, however, the REQ  signal is asserted, the Preset signal is deasserted and an asserted Load pulse is applied to the counter 60 through a synchronous latch 64 and AND gate 66, thereby loading the latency identification value Latency  into the counter.

The counter 60 is a synchronous counter, sampling various input signal states at times referenced to a system clock signal CLK. A reduced frequency clock signal CLK÷M is applied to the decrementing control input DEC of the counter 60 for successively decrementing the latency value produced at the counter output OUT. The reduced frequency clock signal CLK÷M is preferably derived from the system clock signal CLK and has a frequency associated with a typical memory cycle time. For example, if a typical memory cycle requires M cycles of the system clock, the reduced frequency clock signal would have one-Mth the frequency of the system clock. Those skilled in the art will understand that such a reduced frequency clock signal is readily provided by, for example, applying the system clock signal CLK to a conventional “divide-by-M counter,” with the “Carry” output of such counter then providing the reduced frequency clock signal CLK÷M.

The data transfer request signal REQ 1 and the corresponding latency identification value Latency 1 (as well as the clock signals CLK and CLK÷M) are applied to a synchronous down counter and preset/load logic circuitry (not shown) of essentially the same configuration as shown in FIG. 6 and as described above in connection with REQ . The latency values produced at the counter outputs are then applied to the inputs of a first comparator 68 and to the inputs of a multiplexer 70. The first comparator 68 produces an output signal as a function of the comparison of the latency values, which signal is then used as a selection control signal for the multiplexer 70. The multiplexer 70 then passes whichever of the latency values is the lowest value. The signal output by the first comparator 68 also functions as a service priority signal Service /{overscore (Service 1)} with a logic state that indicates which of the requests has current priority for service.

A second comparator 72 compares the latency value passed by the multiplexer 70 to a predetermined value, which may be selected to correspond with a lowest acceptable latency value after which the requested data transfer must be initiated. In the event the lowest latency value output by the multiplexer 70 is less than or equal to this predetermined value, the comparator 72 asserts an override priority signal OVR.

Referring to FIG. 7, a memory control state machine 74 receives the override priority signal OVR, the service priority signal Service /{overscore (Service 1)}, the latency-tagged request signals REQ  and REQ 1, and the conventional priority request signal PREQ. The memory control state machine 74 produces the various well-known memory control signals required to access the main memory 30 (see FIGS. 4 and 5) described above. The memory control state machine 74 also provides the acknowledge pulses ACK , ACK 1, PACK to acknowledge the respective requests and initiate associated data transfer operations.

FIG. 8 is a state diagram depicting the operations performed by the memory control state machine 74 of FIG. 7. Those skilled in the art will appreciate that the particular implementation of the memory control state machine 74 may be accomplished by any of a wide variety of approaches, including a microprocessor or controller executing software instructions, or hard-wired logic circuitry, to name a few examples. In the particular state diagram shown in FIG. 8, REQ  is always given priority over REQ 1, absent override priority. Also, the priority request PREQ is always serviced before the latency-tagged request REQ  or REQ 1, absent override priority. Since the override functionality ensures the latency-tagged data transfer requests will be serviced in a timely fashion, providing a default preference for the conventional priority requests provides regular and timely service of these data transfer requests as well.

Referring to FIG. 8, the Legend identifies the various signal state combinations --D that result in transitions from one to another of the operating states of the memory control state machine 74. The “/” positioned at the beginning of a signal name indicates that the signal is deasserted. The “+” symbol indicates a logic “OR” combination of signal states, and the “·” indicates a logic “AND” combination of signal states, with conventional order-of-operations rules applying. The signal state combination A then corresponds to deasserted REQ  AND deasserted REQ 1 AND deasserted PREQ. The signal state combination B corresponds to deasserted PREQ AND asserted REQ  OR asserted OVR AND asserted SRVC . The signal state combination C corresponds to deasserted PREQ AND deasserted REQ 0 AND asserted REQ 1 OR asserted OVR AND asserted SRVC 1. The signal state combination D corresponds to asserted PREQ AND deasserted OVR.

As shown in FIG. 8, the signal state combination A causes the memory control state machine 74 to transition to or remain in an IDLE operating state 80. The signal state combination B causes the memory control state machine 74 to transition to or continue in a SERVICE REQ  operating state 82, in which the memory control state machine asserts the acknowledge signal ACK  and applies the requisite memory control signals to the memory 30 to provide the requested data transfer. The signal state combination C causes the memory control state machine 74 to transition to or continue in a SERVICE REQ 1 operating state 84, in which the memory control state machine asserts the acknowledge signal ACK 1 and applies the requisite memory control signals to the memory 30 to provide the requested data transfer. The signal state combination D causes the memory control state machine 74 to transition to or continue in a SERVICE PREQ operating state 86, in which the memory control state machine asserts the acknowledge signal PACK and applies the requisite memory control signals to the memory 30 to provide the requested data transfer.

Of course, those skilled in the art will appreciate that the state diagram shown in FIG. 8 can be readily modified to represent the operation of the memory control state machine 74 receiving multiple latency-tagged requests REQ -REQ N and multiple conventional priority requests PREQ -PREQ M. Those skilled in the art will also appreciate that the various signal state combinations causing transition in operating states can be modified during operation of the memory control state machine 74 to accomplish rotating default priorities (i.e., ordering memory accesses absent a latency override priority condition) or other arbitration schemes in addition to the latency-tagged arbitration method and circuitry described above.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. 

What is claimed is:
 1. A computer system, comprising: a memory for storing data; a device operable to assert a data transfer request signal, the device being operable to provide a latency identifier value indicative of a time interval for performing a data transfer operation between the memory and the device responsive to the data transfer request signal; and a memory controller coupling the memory with the device and operable to control data transfer operations therebetween, the memory controller receiving the data transfer request signal and the latency identifier value, the memory controller operable to initiate data transfer operations prior to elapse of the time interval indicated by the latency signal.
 2. A computer system according to claim 1 wherein the memory controller comprises: a counter operable to receive the latency identifier value and a counting control signal, the counter producing a modified latency identifier value in response to the counting control signal; and a comparator operable to receive the modified latency identifier value and to compare the modified latency identifier value with an override value, the comparator operable to assert an override control signal to initiate the data transfer operations.
 3. A computer system, comprising: a memory for storing data; first and second I/O devices, each operable to assert a respective one of first and second request signals and a respective latency identifier, the first and second I/O devices being operable to transfer data to and from the memory; and a memory controller coupling the memory with the first and second I/O devices and operable to control data transfer operations to and from the memory, the memory controller operable to receive respective latency identifiers from the first I/O device and the second I/O device, the memory controller further operable to initiate data transfer operations between the memory and the first I/O device no later than at a time determined by the latency identifier received from the first I/O device, and to initiate data transfer operations between the memory and the second I/O device no later than at a time determined by the latency identifier received from the second I/O device.
 4. A computer system according to claim 3 wherein the memory controller is operable to periodically modify the latency identifier and to compare the latency identifier with a predetermined value, the time at which data transfer operations are initiated being determined by the comparison of the latency identifier with the predetermined value.
 5. A computer system according to claim 4 wherein the memory controller includes a counter for modifying the latency identifier.
 6. The computer system of claim 3 wherein the latency identifier is a first latency identifier, and wherein the me controller is operable to receive a second latency identifier corresponding with the second I/O device, the memory controller comprising a comparator to compare the first and second latency identifiers and correspondingly produce a comparison signal having first and second states, the order of data transfer operations between the memory and the first and second I/O devices corresponding with the first and second states of the comparison signal.
 7. A computer system, comprising: a memory for storing data; a plurality of devices, each operable to assert a respective one of a plurality of data transfer request signals when data is to be transferred between the memory and the device, each of the plurality of devices being further operable to assert a respective latency identifier value corresponding to a time interval for transferring data to or from a respective one of the devices; and a memory controller coupling the memory with the devices and operable to control data transfer operations therebetween, the memory controller operable to receive the data transfer request signals and the plurality of latency identifier values, and wherein the memory controller is further operable to transfer data to or from the devices in a sequence determined by the latency identifier values.
 8. A computer system according to claim 7 wherein the memory controller comprises: modifier circuitry operable to receive the latency identifier values and produce corresponding modified latency identifier values; selection circuitry coupled with the modifier circuitry and receiving the modified latency identifier values, the selection circuitry operable to select a highest priority of the modified latency values; and comparison circuitry coupled with the selection circuitry and receiving the highest priority of the modified latency values, the comparison circuitry operable to compare the highest priority of the modified latency values to a predetermined value and responsively produce a comparison signal, the comparison signal of an asserted state initiating the data transfer to or from the device corresponding with the highest priority of the modified latency values.
 9. A computer system according to claim 8 wherein: the modifier circuitry includes down-count circuitry for successively decrementing the latency identifier values; the selection circuitry is operable to select the lowest of the decremented latency identifier values; and the comparison circuitry produces the comparison signal of the asserted state when the lowest of the decremented latency identifier values is less than the predetermined value.
 10. In a computer system having a memory for storing data and a device operable to transfer data to or from the memory, a memory controller coupling the device and the memory and comprising: a latency counter circuit operable to receive a latency identifier value corresponding with one of the devices and to produce a modified latency identifier value; a comparator circuit operable to receive the modified latency identifier value, compare the modified latency identifier value with a predetermined value, and to produce a comparison signal; and a control state circuit operable to receive the comparison signal and to responsively initiate data transfer operations between the memory and the one of the devices. 